1. Field of the Invention
The present invention relates to motorcycles.
2. Description of the Related Art
Conventionally, an internal combustion engine for motorcycles in which a cylinder head and a cylinder body are formed separately from each other has been known. The cylinder head and the cylinder body are secured to each other by bolts inserted in bolt insertion holes that are formed in their mounting surfaces. Connection of the cylinder head and the cylinder body requires high precision because it greatly affects the performance of the internal combustion engine. For this reason, dowel pin holes, in which dowel pins configured to perform positioning are to be inserted, are formed in the mounting surfaces of the cylinder head and the cylinder body.
In addition, various members including the cylinder head and the cylinder body have an oil passage through which oil for lubricating the components disposed in these member flows. Since it is necessary to form the bolt insertion holes, the dowel pin holes, and the oil passage separately in the mounting surfaces, the size of the mounting surfaces tends to become large. The size increase of the mounting surfaces leads to an increase of the weight of the internal combustion engine. Japanese Examined Utility Model Publication No. H02(1990)-020416 discloses an internal combustion engine in which hollow positioning pins and oil passages are disposed coaxially with each other.
However, in the technique disclosed in Japanese Examined Utility Model Publication No. H02(1990)-020416, sufficient space for forming the hollow positioning pins and the oil passages is required in the regions to the right of the rightmost cylinder, between the cylinders, and to the left of the leftmost cylinder, because the hollow positioning pins and the oil passages are disposed coaxially. As a consequence, the size of the mounting surface of the cylinder body tends to become large.